Steel alloy for a low-alloy high-strength steel

ABSTRACT

A low alloy high-strength carbide-free bainitic steel for producing strips, sheets and pipes is disclosed with the following chemical composition (in weight %) 0.10-0.70 C, 0.25-4.00 Si, 0.05-3.00 Al, 1.00-3.00 Mn, 0.10-2.00 Cr, 0.001-0.50 Nb, 0.001-0.025 N, max 0.15 P, max 0.05 S, remainder iron and steel tramp elements with optional addition of one or more elements of Mo, Ni, Co, W, Nb, Ti, or V and Zr and rare earths with the proviso the for avoiding primary precipitations of AlN the condition Al×N&lt;5×10 −3  (weight %) and for suppressing the cementite formation the condition Si+Al&gt;4×C (weight %) are satisfied.

The invention relates to a steel alloy for a low alloy high strengthsteel which at the same time is tenacious and has excellent wearresistance according to patent claim 1.

In particular, the invention relates to pipes, strips and sheets made ofthis alloy, from which for example components for the automobileindustry such as vehicle bodies, components of support structures orairbag tubes and cylinder tubes are produced. In the field ofconstruction-machine industry, wear plates made of this alloy can forexample be used in case of high wear requirements for excavator shovels.Such steels are also used for applications where sudden impact energieshave to be absorbed, for example as bullet proof armor.

Pipes produced from this alloy can be configured as welded pipes thatare produced from hot or cold strip or in a seamlessly, and whichdepending on the case can have a cross section which deviates from thecircular shape.

Construction pipes or plates made of this alloy can also be used forwelded steel constructions that are exposed to particularly high stressfor example in crane construction, bridge construction, hoistconstruction and heavy-duty vehicle construction.

The demands for ever higher strength and improved processing propertiesand component properties while at the same time reducing weight and/orcosts have lead among other things to the development of ultra-finegrained duplex steels which are also known under the name “SuperBainite” as carbide-free steels. The generation of such a microstructureconsisting of bainitic ferrite with residual austenite lamellae isschematically shown in FIG. 1 in contrast to the upper and lower bainitemicrostructure.

Characteristic for these steels is for example a strength of 1000 toabout 2000 MPa, an elongation at brake of at least 5% depending on thestrength and an extremely fine (nano) structured bainitic microstructurewith portions of residual austenite.

The approach for generating this ultra-fine microstructure is based onthe phase transformation at low temperatures in the bainite region whileavoiding the precipitation of cementite and formation of martensite.Suppression of carbides that precipitate in the bainite such ascementite is necessary because on one hand, these have a strongembrittling effect as possible fracture inducers thereby preventingachieving the required tenacity, and on the other hand, the proportionsof stabilized austenite, which are necessary for achieving theproperties according to the invention, cannot be established.

The economic use of these steels is impeded, however, because at theselow transformation temperatures the transformation kinetic is stronglydecelerated which depending on the alloy composition, in particular withincreasing carbon content, can lead to longer isothermal holding timesof many hours up to one or more days. Such long processing times are,however, not acceptable for an economic production of components so thatalloy concepts were pursued as solutions to accelerate thetransformation.

An alloy composition which requires such a long isothermaltransformation time of up to 48 hours is known from WO 2009/0-75494. Itis also disadvantageous that this steel contains expensive nickel,molybdenum, boron and titanium beside carbon and iron and the achievabletenacities are not yet sufficient for the described fields ofapplication.

Carbide-free bainitic steels for rail tracks are for example known fromDE 696 31 953 T2. Beside manganese, chromium and further elements suchas molybdenum nickel vanadium wolfram titanium and boron, the steelalloy disclosed there has a silicon content between 1 and 3%.

This publication also mentions that beside silicone the addition ofaluminum can reduce or suppress the formation of carbides in the bainiteand can stabilize the remaining residual austenite. This steel alsoallows overcoming the disadvantage of a long transformation time,wherein a corresponding bainitic microstructure can be generated bycontinuous cooling at air (air hardening) alone.

This steel is configured for the demands on rail tracks that are exposedto strong wear stress, however it cannot be used or is uneconomical forstrips, sheets and pipes for the mentioned field of application becausein these cases beside the demands on wear resistance, the strength andtenacity requirements also have to be met. In addition, due the theircompact cross section, the cross sectional dimensions of railssignificantly differ from those of strips, sheets and pipes whichrequires adjustment of the alloying concept with regard to the materialproperties to be achieved after air cooling of the steel. A disadvantageof the known steel is also the expensive addition of titanium and otheralloy elements such as nickel, molybdenum and wolfram.

A further problem in the known steels is that no information regardingthe nitrogen content are given which adversely affects materialproperties in particular through formation of aluminum nitrides whenaluminum is added.

As a result of the addition of aluminum due to the great affinity to thenitrogen present in the steel, coarse aluminum nitrides are formedduring solidification which precipitate primarily in the steel which hasa very negative effect on the ductility notch impact toughness thebursting behavior and the service life of the steel and with thissignificantly deteriorates the mechanical properties.

As a result, this known steel alloy in which instead of silicone,aluminum or in addition further aluminum is added, is rendered unusablein praxis because the amount of precipitations and size of deleteriousaluminum nitrides is dependent on the respective nitrogen and aluminumcontent in the steel and, because the nitrogen is not taken intoaccount, the concrete material properties cannot be predicted. Inaddition, the achievable tenacities for the described field ofapplication according to the invention are also not sufficiently high.

The demands on the mechanical properties of the steel alloy that have tobe satisfied can be summarized as follows:

Strength: 1250 to 2500 MPa

Elongation at break above 12%Notch impact toughness at −20° C.: at least 15 J

Object of the invention is to set forth a steel alloy for a low alloy,high-strength carbide-free bainitic steel which is tenacious and wearresistant for producing strips, sheets and pipes, which on one hand ismore cost effective than the known steel alloys and on the other handensures uniform material properties which meet the demands such asstrength, elongation at break, tenacity etc. In addition, these materialproperties are also to be achieved when cooling at stationary air by airhardening.

This object is solved based on the preamble of claim 1 together with thecharacterizing features. Advantageous refinements are the subject matterof the sub claims.

According to the teaching of the invention, a steel alloy with thefollowing chemical composition is proposed (in weight %):

0.10-0.70 C 0.25-4.00 Si 0.05-3.00 Al 1.00-3.00 Mn 0.10-2.00 Cr0.001-0.50 Nb 0.001-0.025 N max. 0.15 P max 0.05 S

remainder iron with smelting related contaminations with optionaladdition of one or more elements of Mo, Ni, Co, W, Nb, Ti, or V and Zrand rare elements with the proviso that for avoiding primaryprecipitations of AlN the condition Al×N<10⁻³ (weight %) and forsuppressing cementite formation the condition Si+Al>4×C (weight %) aresatisfied.

Optionally rare earths and reactive elements such as Ce, Hf, La, Re, Scand/or Y of a\overall up to 1 weight % can be added.

In the puddle lump or slab state, steels according to the invention havealready after cooling at air a strength (R_(m)) of over 1250 MPa, anelongation at break of over 12% and a tenacity (KBZ) at −20° C. of atleast 15 J (cf. Table 1). The microstructure consists of carbide-freebainite and residual austenite with a proportion of at least 75%bainitic ferrite, at least 10% residual austenite and up to maximally 5%martensite (or martensite phase and/or decomposed austenite).

The steel alloy according to the invention is based on the developmentof the carbide-free bainitic steel form DE 6906 953 T2 and WO2009/075494 A1.

Tests that were carried out in the context of the present invention havesurprisingly shown that compared to known steel alloys for achieving thedemanded material properties already can be achieved by an air hardeningby targeted addition of aluminum in the range of 0.05 to 3.0 weight %and niobium in the range of 0.001 to 0.5 weight % beside an excellentmaterial strength and wear resistance, very good tenacity can beachieved. In particular, the addition of niobium results in asignificant improvement of the tenacity properties through grainrefinement, so that this alloy meets the high requirements regardingmechanical properties and wear resistance.

Also, as a result of the advantageous addition of chromium in the rangeof 0.10 to 2.00 weight %, the kinetic of the ferrite formation can bedecisively controlled so that the formation of coarse polygonal ferritebodies, which can adversely affect the material properties, can beeffectively avoided. Important in this regard is the interaction betweenaluminum and chromium. While aluminum accelerates the ferritic andbainitic transformation, addition of chromium delays the ferritictransformation (cf. FIG. 2). Targeted combination of these two elements,allows controlling the kinetic of the ferrite and bainite formation.

Beside the known advantageous effect of adding aluminum on avoidingcarbide precipitations in the bainite, tests have shown that theaddition of aluminum compared to silicone significantly accelerates thekinetic of the bainitic transformation. The latter also increases withincreasing contents of aluminum which mean that the tenacity andstrength of the steel according to the invention is significantlyimproved after continuous cooling compared to steel which are onlyalloyed with silicone, i.e., higher tenacity and strength values can beachieved. Advantageous are cooling rates greater than 10° C./s in orderto achieve the demanded combination of mechanical properties also inthicker sheets (for example above 10 mm); the demanded mechanicalproperties can also be achieved by means of cooling at stationary air inthe case of thinner sheets or by adjusting the alloying concept. Theinfluence of different alloying elements on the kinetic of thetransformation is shown in FIG. 2. The effects of C, Si, Al, Mn, Cr andMo on the transformation kinetic of ferrite, perlite and bainite and onthe martensite start temperature are shown schematically.

According to the invention, compared to the known steel, it is strictlynecessary for achieving these advantageous properties that the nitrogencontent does not exceed the stated upper limit of 0.025%, better 0.015%or optimally 0.010 weight % in order to minimize the number and size ofthe deleterious aluminum nitrides as primary precipitations in thesteel, wherein in addition the condition Al×N<5×10⁻³ has to besatisfied. Otherwise, a minimal content of nitrogen of 0.001 weight %,optimally 0.0020 is required in order to enable a required niobiumcarbonitride formation for increasing tenacity by grain refinement.

The tested alloy compositions and the determined mechanicalcharacteristics are shown in Table 1. All samples where heated to about950° C. and then cooled at stationary air or subjected to acceleratedcooling. The required cooling speed is selected depending on the sheetthickness and the composition. As the results of the mechanical samplingshow, the demanded properties could not be achieved with the sample melt14 due to the too low Cr content. The test melt 16 satisfied the demandsdue to the greater sheet thickness of 12 mm only after acceleratedcooling. Typical temperature profiles for the cooling at stationary airor with quenching are shown in FIG. 3.

In FIG. 4 some of the tested test melts and their mechanicalcharacteristics and cooling conditions are shown in comparison to theconventional and high strength steel materials. It can be seen that inthe developed steel the region of higher strength materials at improvedstretch properties.

TABLE 1 Alloy compositions in weight % and mechanical charateristicvalues of the tested alloys Element content/weight % C Si Mn P S Al CuCr Ni Mo V Ti Nb Co B W N Test steels 16 0.299 1.110 1.975 0.015 0.0020.107 — 0.751 — 0.105 — — 0.021 — — — 0.003 according 17 0.396 3.0401.938 0.016 0.003 0.280 — 0.751 — 0.104 — — 0.020 — — — 0.005 to theinvention Test steels 05 0.197 1.260 2.230 0.012 0.002 0.500 — 0.082 —0.260 — — — — — — 0.003 not-according 14 0.231 1.080 1.937 0.013 0.0020.092 — 0.098 — 0.105 — — 0.020 — — — 0.004 to the 16 0.299 1.110 1.9750.015 0.002 0.107 — 0.751 — 0.105 — — 0.021 — — — 0.003 invention SheetCooling Mechanical properties thickness/ rates/ Rp0.2 Rm Ag A KBZ/J mmC. °/s. /MPa /% @ RT @ 0 @ −20 @ −40 @ −60 Test steels 16 12.0 025 10211670 13.2 19.8 69 56 48 42 30 according 210 1214 1531 02.3 13.3 52 35 3425 18 to the 17 12.0 020 1254 1648 02.3 12.3 31 25 15 11 08 invention190 1104 1884 08.1 13.7 39 25 20 15 12 Test steels 05 12.5 001 0760 109408.5 19.3 47 27 not-according 14 09.0 030 0578 1136 13.3 17.2 16 15 12to the 16 12.0 001 0796 1352 11.3 18.0 16 10 08 08 invention

The results confirm the excellent mechanical properties (strength andtenacity of the steel alloy according to the invention already forsemi-finished products such as puddle lumps or slabs) in the hardenedstate (Table 1).

As essential element, aluminum plays an important role which besideaccelerating the transformation kinetic also suppresses the carbideprecipitation in the bainite in combination with silicone, as a result,residual austenite is stabilized because carbon only has a limitedsolubility in the ferrite. A high proportion of residual austenite of atleast 10% in the bainite causes beside the extremely fine lamellarmicrostructure the excellent mechanical properties. The differentmicrostructure components were determined by scanning electronmicroscopy, wherein a mean lamellar interspacing of 300 nm wasdetermined. A schematic representation of a previous austenite grainwith substructure (such as for example sub grains) with fine lamellarmicrostructure is schematically shown in FIG. 5. Here, the previousaustenite grain structure is stabilized via Nb(C,N) precipitations.

With corresponding proportions of residual austenite a so calledTRIP-effect can then also be advantageously used. Steels which usuallyare referred to by the term TRIP (“Transformation Induced Plasticity”)are steels which at the same time have a very high strength and a highductility, which makes them especially suited for cold forming. Theseproperties are obtained owing to their special microscopic structure,wherein the deformation-induced martensite formation and the workhardening associated therewith is inhibited and the ductility isincreased. The effect of the TRIP effect is optimal for a residualaustenite proportion of about 1 to 20%.

In the following, the alloying concept according to the invention isexplained in more detail.

-   -   Carbon: for reason of sufficient strength of the material the        minimal content should not be below 0.10 weight %. With regard        to a sufficiently low martensite start temperature and with this        the establishment of a very fine microstructure, however still        good weldability, the carbon content should not be above 0.70        weight %. Carbon contents between 0.15 and 0.60 weight % have        proven advantageous, wherein optimal properties are achieved        when the carbon content is between 0.18 and 0.50 weight %.    -   Aluminum/silicone: the essential element for achieving the        demanded material properties after continuous cooling is        aluminum which strongly accelerates the transformation kinetic.    -   In order to achieve this effect the aluminum content should be        at leas 0.05% weight % but maximally 3.00% weight % because        otherwise coarse polygonal ferrite bodies could form which in        turn adversely affect the material properties. If the aluminum        content is too low the bainitic transformation becomes too slow        so that the formation of martensite is promoted which adversely        affects the elongation at break and the notch impact toughness.        For a sufficient suppression of carbides in the bainite,        silicone at contents of 0.25 to 4.00 weight % can be added. Good        material properties are achieved at aluminum contents between        0.07 and 1.50 weight % and optimal ones between 0.09 and 0.75        weight %. Corresponding silicone contents are between 0.50 to        1.75 weight % or between 0.75 and 1.50 weight %.    -   As a result of the targeted addition of chromium of at least        0.10 to 2.00 weight % the ferritic transformation can be        decelerated and via a combination with aluminum the kinetic of        the ferrite and bainite formation can be controlled in a        targeted manner. Advantageous chromium contents are between 0.10        to 1.75 weight % or between 0.10 and 1.50 weight %.    -   Manganese: the addition of Manganese in the range of 1.00 to        3.00 weight % results in dependence on the respective demands on        the steel alloy as a compromise between strength, which can be        achieved by higher additions, and a sufficient tenacity which        can be achieved at lower contents. With regard to a very good or        optimal property combination, the manganese content should be        between 1.50 and 2.50 weight % or between 1.70 and 2.70 weight        %.    -   Niobium/nitrogen: A niobium content of 0.001 to 0.50 weight %        has to be established for ensuring formation of Nb(C,N). The        resulting grain refinement contributes to a significant        improvement of the tenacity properties. In addition a nitrogen        content of 0.001 to 0.025 weight % is recommended for forming        Nb(N) because NbN is more stable than NbC and thus leads to an        increased grain refinement. Advantageous niobium contents are        0.001 to 0.10 or 0.001 to 0.05 weight % at advantageous nitrogen        contents of 0.001 to 0.015 or 0.002 to 0.010 weight %. In        addition, adding nitrogen prevents excessive binding of C via Nb        because otherwise the austenite stabilizing effect of C could be        lost.    -   If needed for example molybdenum (up to 1.00 weight %), nickel        (up to 5.00 weight %) cobalt (up to 2.00 weight %) or wolfram        (up to 1.50 weight %) can be added as solid solution hardener        for further increasing strength. As an alternative or in        addition micro-alloying elements such as vanadium at up to 0.2        weight % and/or titanium up to 0.10 weight % can be added. A        total content of Ti, V of max 0.20 weight % and Ni, Mo, Co, W,        Zr of max 5.50 weight % should be observed. In order to take        advantage of the effect of these alloy elements, a minimal        content of 0.01 weight % should be observed.    -   Rare earths and reactive elements: rare earths and reactive        elements such as Ce, Hf, La, Re, Sc and/or Y can be optionally        added to achieve an optimal lamellar interspacing and thus for        further increase of strength and tenacity at total contents of        up to 1 weight %. If necessary a total content of 20 ppm should        be added.

In the alloy composition the following conditions should be adhered tofor achieving the demanded material properties in particular of themechanical technological properties for the transformation kinetic andthe transformation behavior (FIG. 2) the stabilizing of the residualaustenite and the martensite start temperature while taking the coolingrate into account wherein in the mentioned empirically determinedformulas the contents of C, Mn, Si, Al, Cr and Mo in weight % and T ascooling rate in ° C./s have to be inserted. The units of thecoefficients that are used in the formula are to be selected accordingto the variables used in the formula.

Kinetic of the Ferritic Transformation:

For adhering to or setting the mechanical-technical properties and inparticular for avoiding the formation of the coarse polygonal ferritegrains which adversely affect the material properties, the followingcondition has to be satisfied:

(35×C)+(10×Mn)−Si−(5×Al)+Cr>134/{dot over (T)}+10

Kinetic of the Bainitic Transformation:

The following equation for the kinetic of the bainitic transformationhas to be established to achieve a suitable microstructure with veryfinely configured bainitic ferrite/residual austenite lamellae for themechanical technological properties:

400×exp[(−7×C)−(4×Mn)+8Al+3]/{dot over (T)}>1

Martensite Start Temperature (° C.):

for avoiding greater martensitic microstructure proportions whichdeteriorate the mechanical technological properties the martensite starttemperature has to be determined as follows:

525−(350×C)−(45×Mn)−(16×Mo)−(5×Si)+(15Al)<<400

For stabilizing the residual austenite, formation of cementite has to besuppressed. This is achieved by a targeted alloying with Si and Albecause both elements have a very low solubility in cementite. For this,the following condition has to be satisfied:

Si+Al>43×C

For avoiding deleterious primary AlN precipitations, the followingcondition has to be satisfied:

Al×N>5×10⁻³

In FIG. 6, this relationship is again shown schematically.

Transformation Capacity:

For establishing the properties according to the invention on the basisof the describe microstructure a complete austeniziation of the steelsaccording to the invention has to be achieved prior to the final heattreatment (cf. FIG. 1).

In order to achieve the demanded combination of the mechanicalproperties (strength ductility and tenacity) the following relationshipof the ferrite and austenite formers is to be satisfied:

C+Si/6+Mn/4+(Cr+Mo)/3>1.

The microstructure of the steel according to the invention consist offerrite and residual austenite lamellae. It can have proportions ofmartensite of up to 5% (or martensite/austenite phase and/or decomposedaustenite). The two most important characteristics of the microstructurewhich significantly influence the mechanical properties of the steel arethe lamella spacing and the proportion of residual austenite. Thesmaller the lamellar interspacing and the higher the proportion ofresidual austenite the higher are the strength and elongation at breakof the material.

In order to achieve the demanded high strength of the material of atleast 1250 to 2500 MPa the average lamellar interspacing should besmaller than 750 nm, advantageously smaller than 500 nm.

In order to achieve the stretch values of at least 12% (and elongationat break) a residual austenite proportion of at least 10% and amartensite proportion of at most 5% should be present.

In order to achieve the high tenacity by grain refinement by means ofniobium carbonitride formation, the average previous austenite grainsize should not exceed a value of 100 μm.

Because the microstructure is very fine, the components of themicrostructure can hardly be distinguished from each othermicroscopically so that depending on the case a combination of electronmicroscopy and x-ray diffraction has to be used.

The components of the microstructure can be distinguished by means ofscanning electron microscopy. In this way, an average lamellarinterspacing of about 300 nm was determined.

The result of an x-ray diffraction measurement is shown in FIG. 7. Fromthe intensity distribution of the x-ray spectrum, the crystal structureof the present microstructure components and their phase proportions canbe determined.

Residual austenite proportions between 10% and 20% were determined usingthe x-ray diffraction method.

What is claimed is:
 1. A steel alloy for a low alloy high-strengthcarbide-free bainitic steel for producing strips, sheets and pipes withthe following chemical composition (in weight %): 0.10-0.70 C 0.25-4.00Si 0.05-3.00 Al 1.00-3.00 Mn 0.10-2.00 Cr 0.001-0.50 Nb 0.001-0.025 Nmax 0.15 P max 0.05 S remainder iron and steel tramp elements withoptional addition of one or more elements of Mo, Ni, Co, W, Nb, Ti, or Vand Zr and rare earths with the proviso the for avoiding primaryprecipitations of AlN the condition Al×N<5×10⁻³ (weight %) and forsuppressing the cementite formation the condition Si+Al>4×C (weight %)is satisfied.
 2. The steel alloy according to claim 1, having thefollowing contents in weight %: 0.15-0.60 C 0.50-1.75 Si 0.07-1.50 Al1.50-2.50 Mn 0.10-1.75 Cr 0.001-0.10 Nb 0.001-0.015 N
 3. The steel alloyaccording to claim 2, having the following contents in weight %:0.18-0.50 C 0.75-1.5 Si 0.09-0.75 Al 1.70-2.50 Mn 0.10-1.5% Cr0.001-0.05 Nb 0.002-0.010 N
 4. The steel alloy according to claim 2,having the following contents in weight percent: max. 5.00 Ni max. 1.00Mo max. 2.00 Co max. 1.50 W max. 0.10 Ti max. 0.20 V wherein the totalcontent of Ti, V is maximally 0.20% and the total content of Ni, Mo, Co,W is maximally 5.50 weight %.
 5. The steel alloy according to claim 1,wherein the microstructure consists of carbide free bainite and residualaustenite with a proportion of at least 75% bainite at least 10%residual austenite and up to maximally 5% martensite.
 6. The steel alloyaccording to claim 1, wherein for achieving demanded material propertiesthe following conditions for transformation kinetic martensite starttemperature and microstructure formation are adhered to: ferritictransformation kinetic with C, Mn, Si and Al corresponding to theelement contents in weight % and T to a cooling rate in C/s:(35×C)+(10×Mn)−Si−(5×Al)+Cr>13/{dot over (T)}+10 bainitic transformationkinetic with C, Mn and Al corresponding to the element contents inweight % and T to the cooling rate in C/s:400×exp[(−7×C)−(4×Mn)+8Al+3]/{dot over (T)}>1 martensite starttemperature with, C, Mn, Si Al and Mo correspond to the element contentsin weight %:525−(350×C)−(45×Mn)−(16×Mo)−(5×Al)<<400 stabilization of residualaustenite with C, Si and Al corresponding to the element contents inweight %):Si+Al>4×C avoiding primary AlN precipitations with Al and Ncorresponding to the element contents in weight %:Al×N<5×10⁻³ satisfying the demanded combination of the mechanicalproperties:C+Si/6+Mn/4+Cr+Mo)/3>1
 7. The steel alloy according to claim 1,characterized in that an average distance of residual austenite lamellasis less than 750 nm.
 8. The steel alloy according to claim 7, whereinthe average distance of the residual austenite lamellas is less than 500nm.
 9. The steel alloy according to claim 1 for use in production of hotor cold rolled strips sheet metals pipes profiles or for forged partsfor the automobile industry, construction industry and machineconstruction and rods and wires.
 10. The steel alloy according to claim1 for use in wear parts and parts for armors.